This invention relates generally to thermodynamics in gas turbine engines and more particularly to apparatus for extracting bleed air in such engines.
A gas turbine engine includes a turbomachinery core having a high pressure compressor, combustor, and high pressure turbine in serial flow relationship. The core is operable in a known manner to generate a primary flow of propulsive gas. A typical turbofan engine adds a low pressure turbine driven by the core exhaust gases which in turn drives a fan rotor through a shaft to generate a bypass flow of propulsive gas. In the case of a high bypass engine this provides the majority of the total engine thrust.
A typical axial flow high pressure compressor in such an engine includes a number of stages. Each stage has a row of rotating airfoils or blades and row of stationary airfoils or vanes. The vanes serve to turn the airflow exiting an upstream row of blades before it enters the downstream row of blades. It is known to construct one or more rows of vanes so that their angle of incidence can be changed in operation. These are referred to as variable stator vanes or simply “VSVs”. The VSVs enable throttling of flow through the compressor so that it can operate efficiently at different flow rates, without the losses incurred by other mechanisms such as bleed valves. Because of high overall pressure ratios and stage count in many compressors, there will often be many stages of VSVs.
It is known to extract high-pressure compressed air from the high pressure compressor. This referred to as “bleed air” and may be used for purposes such as engine or aircraft anti-icing, boundary layer control devices, aircraft environmental control systems and the like. For optimal engine performance, bleed should occur at the stage that provides the minimum source pressure the user requires. However, in the prior art, sources have been limited to stages aft of the last VSV stage, because of the structural difficulty of extracting air from the VSV stages. Thus, the only conveniently available bleed source is at an undesirably high pressure.
Accordingly, there is a need for a compressor which allows air to be bled from the VSV stages.